Recently, in response to realized application of a blue-light emitting diode, a white-light emitting diode having this diode as a light source is actively studied. Since the white-light emitting diode is lightweight, does not use mercury, and has long lifetime, a rapidly rising demand is expected for it in future. In general, as a white-light emitting diode, those obtained by coating paste of a mixture containing YAG (Y3Al5O12:Ce) powder activated by cerium and epoxy resin on a blue-light emitting device are used (Patent Literature 1).
However, as the fluorescent color of YAG:Ce is present near x=0.41, y=0.56 of CIE color coordinates, when admixed with excitation light with blue color of 460 nm, the color is controlled on a line which connects the color coordinates of the blue-light emitting diode and the color coordinates of YAG, and as a result, white color mixed with greenish blue color is obtained instead of white color. For such reasons, a problem arises in that only white color with insufficient red color can be achieved. To deal with such poor color problem, YAG:Ce phosphor powder is admixed with a separate phosphor powder exhibiting red color for color control.
However, there are only very few reports regarding a phosphor which absorbs blue-color light and emits red-color fluorescence. As a specific example, Ba2Si5N8 activated by europium (Eu) has been reported in Non-Patent Literature 1. Further, CaAlSiN3 activated by Eu, which exhibits higher light output than Ba2Si5N8, is recently found (Non-Patent Literature 2). Further, Patent Literature 2 discloses an example in which a powder of crystalline silicon nitride having average particle diameter of 0.5 μm and oxygen content 0.93% by weight is used as a raw material and compositional ratio and sites for CaAlSiN3 are modified to control florescence characteristics.
Further, as a phosphor having A2Si5N8 crystal structure that is activated rare earth element, a red-light emitting material as disclosed in Non-Patent Literature 3, i.e., a phosphor having MxSiyNz (M=Ca, Sr, Ba, or Zn; x, y, and z may have various values) as a mother skeleton, or Patent Literature 3, i.e., a phosphor having MxSiyNz:Eu (M=Ca, Sr, Ba, Zn; z=2/3x+4/3y) as a mother skeleton is known. However, with excitation by blue-color visible light, the red light emission intensity remains insufficient.
Recently, a white-light emitting diode having near ultraviolet to blue-light emitting diode as an excitation source is actively studied. Thus, a phosphor emitting strong yellow light in response to such excitation wavelength is waited for. As a yellow-color luminescent material, Y3Al5O12:Ce has been conventionally used. However, as disclosed in Patent Literature 4, La3Si6N11 is also known.
Accordingly, those nitride phosphors can be used for a vacuum fluorescent display (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode ray tube (CRT), a white-light emitting diode (LED), or the like, and their use as a phosphor having low luminance degradation is expected.